An investigation has been made into the possibility of observing optical transitions (in the 100-micron region) between the ground state of a donor impurity In silicon and the remaining five states of the {1s} set introduced by Kohn and Luttinger. While such transitions are forbidden in the usual effective mass approximation, it is found that application of corrections to the effective-mass wave functions leads to an enhanced transition probability. Under- the assumption of a simple cubic lattice of impurities, the calculated absorption coefficient is of the order of 10 cm[power -1] at an impurity concentration of 1 x 10[power 18] cm[power -3], and falls off exponentially with decreasing impurity concentration. An upper limit is placed on the region in which the transition should be observable by the broadening of the 2s-2p impurity band. It is estimated that for concentrations greater than 5 x 10[power 18] cm[power -3] the transition of interest will be obscured. The calculated values of the absorption coefficient are probably only accurate to within, one, or even two, orders of magnitude, because of the approximations involved. However, there would appear to be no firm theoretical reason why the 1s[superscript (0)] → 1s[superscript (5)] transition should not be observed. / Science, Faculty of / Physics and Astronomy, Department of / Graduate

Measurements have been made on a variety of germanium and silicon junction diodes in order to determine departures from the ideal behavior both under illuminated and dark conditions. The diodes included alloyed, grown and diffused junction types and the experimental study was largely confined to forward voltage. Carriers injected by a forward voltage into the bulk regions of a p-n junction in Ge or Si reduce the resistances of these regions, A model of a photodiode consisting of an ideal diode (Shockley, 1949) in series with this carrier modulated resistance produces good agreement with the experimentally observed dark chs.; in one Ge alloy diode the agreement was exact up to at least 0,7V forward voltage. For large forward currents (density of injected carriers comparable to injected carrier density) the Shockley expression relating I and V is no longer valid nor consequently is the conductivity modulation theory. This theory, making use of the experimental dark chs, and the experimental dynamic capacity measurements, is able to deduce many of the parameters of the diode material e.g. Ƭ𝞺, Pո,Ψo. The chs. of the diode dark and illuminated have been found to intersect at large forward currents. Theoretical investigation of the condition for crossover in various models of the diode is made. The physical meaning of this condition is that at some forward voltage the resistance of the diode is sufficiently reduced on illumination to offset the effect of the opposed internal photo-e.m.f.. For an ideal diode the relation between the short circuit photo-current and open-circuit photo-e.m.f. (photochs.) should simply be the same as the dark forward chs., except for a reversal of sign of the current. The fact that the experimental photochs falls below the ideal photochs is attributed to internal resistance in the diode and an expression is developed from this assumption; the theoretical expression is largely consistent with experiment. Several other models are examined for the non-ideal behavior of junction diodes which attempt to take into account the possible effects of the resistance of the bulk material and of the electrode-semiconductor contact; the implications of non-linear recombination are also investgated. It is found that none of these alternative models is as successful as the conductivity-modulation scheme in explaining the dark, the illuminated and the photocharacteristics. / Science, Faculty of / Physics and Astronomy, Department of / Graduate

Microstructuring of silicon is performed to alter its optical and electrical properties for use in photonic devices. Femtosecond lasers are a favourable structuring tool because they are extremely precise due to the confinement of their interaction to the focal volume. Experiments were carried out on N-type, P-type, and intrinsic silicon with a femtosecond laser operating at 800 nm, with pulse duration of 40 fs, and 1 kHz repetition rate. A single pulse produced a micro-ring structure surrounding a crater. It is caused by the motion of material according to the pressure gradient induced by the Gaussian profile of the laser. Multiple-pulse structures were similar to the single pulse except for the central protrusion of material. Two factors are responsible for multiple-pulse structures: (1) geometrical difference of the plasma compared to the single pulse (2) reflections of shockwaves produce protruding structures. Polarization dependence of all structures was observed.

The effect of impurity concentration on the photoconductive spectrum of boron-doped silicon at helium temperatures was investigated. Photoconductivity was observed for excitations of the bound hole into the impurity excited states. The photoconductivity in this region depends strongly on impurity concentration and was interpreted as being conduction through excited state impurity bands. Two bands were observed to form, with excited states 3 and 4 in the boron spectrum forming one band and state 2 forming the other. A qualitative description of the mobility in the impurity bands was obtained using Baltensperger's theory and the additional assumption that the holes are scattered by "randomness" in the impurity array. Photoconductivity of holes in the valence band was also measured in order to obtain lifetimes and capture cross sections. Assuming the mobility in this region to be determined by neutral impurity scattering, the hole lifetime was estimated to be ~ 10⁻⁹ sec and the capture cross section of ionized boron ~ 3 x 10⁻¹⁰ cm². Assuming similar lifetimes for the holes in impurity bands, the peak mobility in band 3 and 4 was found to be ~ 150 cm² /volt-sec. The d.c. characteristics of the various samples, while at low temperature and exposed to room temperature radiation, were measured during the course of this investigation. All samples were observed to have a non-linear dependence between the current and applied field, terminated by a non-destructive low field breakdown. / Science, Faculty of / Physics and Astronomy, Department of / Graduate

In boron doped silicon, optical excitation of bound holes from the ground state to the various excited states of the neutral acceptor impurity leads to an absorption line spectrum. By applying an external strain, the degeneracies of the acceptor ground state and the four lowest "observable" excited states were determined and were found to only partially agree with theory (Schechter 1962). By applying a uniform electric field to compensated samples, the "Stark effect" for the acceptor states was observed. The Stark shift of the excited states is second order in the field as predicted by Kohn (1957) from symmetry considerations. The Stark broadening of the acceptor absorption lines was attributed to an unresolved partial removal of degeneracy of the excited states. The absorption line broadening mechanisms (phonon, dislocation, concentration, ionized impurity) were determined from new halfwidth measurements, which corrected an earlier study (Colbow 1963). The ionized impurity broadening is caused by the screened Coulomb "internal fields" of nearby ionized impurities which are present in uncompensated samples at temperatures greater than 50°K. A new theory of this broadening contribution (Cheng 1966) is in reasonable agreement with experiment; the earlier theory of the same effect (Colbow 1963) is shown to be inadequate. The effects of compensation on the boron absorption spectrum were measured and attributed to the unscreened Coulomb fields of ionized impurities present because of the compensation. The properties of a weak new absorption line which appeared in the compensated spectrum are described. / Science, Faculty of / Physics and Astronomy, Department of / Graduate

In uncompensated phosphorus or arsenic-doped silicon, observation of the shallow, donor absorption line spectrum for donor concentrations from 10(12) to 2 x 10(18)/cc, yields information about the bound states of the donor electrons and the behavior of the conduction band edge. With increasing concentration, the photon induced IS(A) → 2P(o) and IS(A) → y 2P(±) transition lines are observed to broaden in energy, and shift to slightly lower energies; the lineshapes, Lorentzian at low donor concentrations, become asymmetric with the formation of a tail to the low energy side. A superposed absorption background is observed for donor concentrations greater than 2 x 10(16)/cc. Above 2 x 10(17)/cc an absorption edge in the 10-to-30 meV region with an exponential energy dependence is observed. The change in the absorption line spectrum is due primarily to the final states. The halfwidths of the transitions are well explained by a donor-pair model (or effective hydrogen molecule (Macek, 1971)), with the donor distribution assumed random. The difference in broadening of the 2P(±) level compared to the 2P(o) level is due to the anisotropic effective mass of the donor electron. The transition lineshapes are quantitatively explained in terms of the convoluted Fano function (Bhatia, 1970; Fano, 1962). The Fano parameters Q and T, are interpreted in terms of the evidence for the conduction band tailing and the possible configuration interaction. The integrated absorption coefficient of the line spectrum and the decrease of the absorption cross section above the conduction band threshold with increasing concentration are also accounted for in terms of band tailing. The experimental results are in agreement with the infrared absorption data for antimony-doped germanium (Nisida and Horii, 1969). / Science, Faculty of / Physics and Astronomy, Department of / Graduate

The presence of configuration interaction, between the "internal" acceptor states of group III impurities in silicon and P₃⁄₂ band Bloch states, has been observed. The effect of impurity-impurity interaction on the line shape of the boron, gallium and indium internal acceptor lines has been studied. The observed characteristic asymmetric line shape is explained by introducing the interaction of the internal state with the degenerate P₃⁄₂ valence band Bloch states, and the inhomogeneous impurity-impurity interaction. The ionization energy of the substitutional impurity in the host lattice is found to be a deciding factor in this interaction. The internal and external absorption spectra of silicon doubly-doped with boron and indium acceptors is studied. The observed broadening of the boron external absorption lines in Si(B,In) is explained by neutral impurity scattering. The presence of indium in Si(B,In) crystal lattice, modifies the P₃⁄₂ valence band Bloch states and hence the configuration interaction. This modification is found responsible for the observed features of the boron 2p′ internal line in Si(B,In). The coupling between the lattice and the impurity bound carrier for deep monovalent acceptors in silicon, such as gallium and indium, is found to be stronger than for the shallow boron impurity. This suggests the possible existence of phonon-assisted transitions associated with these deep impurities. Such transitions are observed in the absorption spectrum of indium doped silicon. The phonon-assisted transitions are superimposed on the photoionization continuum transitions of the indium acceptors. Interference effects between the phonon-assisted transition and the transitions to the continuum states modify the position and line shape of the transitions. Using the phonon dispersion curves for silicon, interpretation of the results is presented. As a supplementary study, the temperature dependence of the indium external line 2, is investigated. The temperature dependence of halfwidth of indium line 2 in Si(In) supports the stronger electron-phonon coupling in Si(In). / Science, Faculty of / Physics and Astronomy, Department of / Graduate

The e.p.r. spectrum for relatively dilute samples of phosphorus-doped silicon (<5 x 10(16) donors/cm³) has been calculated in detail for an assumed random distribution of impurities. The system of donor electron spins is treated as a collection of nearest neighbor donor pairs. An expression is derived for the donor pair exchange energy using Kohn-Luttinger wavefunctions and a general exchange energy expression. The resultant relationship contains an adjustable parameter a*, the "effective Bohr radius", which is determined from a comparison of the calculated spectrum and the experimental results obtained for the ratio, C, of the "central pair" and "hyperfine" line intensities. The resulting expression J(R), where J represents the exchange energy and R the separation vector connecting the two pair donors, exhibits an oscillatory spatial dependence due to interference from portions of the wavefunction arising from different conduction band valleys. / Science, Faculty of / Physics and Astronomy, Department of / Graduate

The kinetics of the free exciton, bound exciton and multi-exciton complexes have been investigated for Si(B) by using the rate equations to analyze the experimental transient responses of these luminescent species. From this analysis, the B capture cross section for an exciton to bind upon a neutral impurity has been determined, as well as the B capture cross sections ratios for higher order complexes. Furthermore, the lifetimes for the BE, BMEC-2 and BMEC-3 have been determined. From the low temperature lifetimes, the BMEC-m/BE lifetime ratio appears to form a simple integer series for increasing m. The computer programs used to analyze the experimental data and generate the theoretical transient responses, along with technical information concerning the experiment, are listed in the appendices. / Science, Faculty of / Physics and Astronomy, Department of / Graduate

Graphene has attracted significant research attention for next generation of semiconductor devices due to its high electron mobility and compatibility with planar semiconductor processing. In this dissertation, the influences of Ohmic metals and high dielectric (high-k) constant aluminum oxide (Al2O3) deposition on the structural and electrical properties of multi-layer epitaxial graphene (MLG) grown by graphitization of silicon carbide (SiC) substrates have been investigated. Uniform MLG was successfully grown by sublimation of silicon from epitaxy-ready, Si and C terminated, 6H-SiC wafers in high-vacuum and argon atmosphere. The graphene formation was accompanied by a significant enhancement of Ohmic behavior, and, was found to be sensitive to the temperature ramp-up rate and annealing time. High-resolution transmission electron microscopy (HRTEM) showed that the interface between the metal and SiC remained sharp and free of macroscopic defects even after 30 min, 1430 °C anneals. The impact of high dielectric constant Al2O3 and its deposition by radio frequency (RF) magnetron sputtering on the structural and electrical properties of MLG is discussed. HRTEM analysis confirms that the Al2O3/MLG interface is relatively sharp and that thickness approximation of the MLG using angle resolved X-ray photoelectron spectroscopy (ARXPS) as well as variable-angle spectroscopic ellipsometry (VASE) is accurate. The totality of results indicate that ARXPS can be used as a nondestructive tool to measure the thickness of MLG, and that RF sputtered Al2O3 can be used as a (high-k) constant gate oxide in multilayer grapheme based transistor applications.